Optical Time Domain Reflectometers (OTDRs) are commonly used to characterize an optical fiber. Their use permits single-ended measurements that allow measurements to be undertaken with only one field technician, thereby reducing the expense associated with additional testing personnel. Typically an OTDR can provide total loss, length and return loss of a fiber, as well as localize loss and reflectance at each joint (splice or connector). In order to characterize the input and output connectors of a fiber optic link, it is usual to add a proper lead-in (“launch”) fiber and a termination fiber (sometimes referred to as a “receive” fiber) in order to provide a reference backscattering level before and after each connector. This method is well known by the users of OTDR.
It is frequently required, such as in a Fiber-To-The-Home (FTTH) passive optical network (PON), to test a cable that contains many fibers and to ensure that, at the distal end of the cable under test, the constituent fibers are correctly identified for subsequent connection to a patch panel or fiber distribution hub (FDH). The fiber distribution hub normally comprises at least one 1×N (where N is frequently equal to 32) passive splitter, to which N different fibers may be attached. The cable-under-test generally comprises a subset (e.g. 4) of these N different fibers and terminates at a terminal drop box (DB), from whence the OTDR measurement is launched.
It is known, when testing a cable that contains many fibers, to connect the OTDR to each fiber-under-test (FUT) in turn using a single launch fiber, while the distal end of each of the fibers-under-test (FUTs) remains connected to the FDH or patch panel. However, this approach is not always feasible in an active (i.e. operational/in service) passive optical network (PON), since the launched OTDR light may interfere with system operation.
An alternative known approach, when testing a cable that contains many fibers, is to connect the OTDR to each fiber-under-test (FUT) in turn using a single launch fiber, while the distal end of each of the fibers-under-test (FUTs) is connected to a respective one of a series of termination fibers. While a single launch fiber will usually suffice because the user can easily move it from one fiber input to each of the others, it is preferable to use a series of termination fibers because it obviates the need for the user to go to the FDH and back to the terminal drop box, between measurements, to move the termination fiber from the distal end of one FUT to the next.
It is often desirable to be able to determine which termination fiber is detected on the OTDR trace. The mapping between the ends of the FUTs and the termination fibers allows determination of which distal fiber end corresponds to a given input fiber. This feature is very useful since it can detect errors in fiber marking and cable deployment.
One approach is to use a multi-fiber receive box wherein each of the termination fibers is of a different length, this difference in length being sufficiently large so as to be readily resolvable by the OTDR. The reflection from the end of each termination fiber, whether arising from the approximately 4% Fresnel reflection at a perpendicularly-cleaved glass-air interface or from some explicit reflective element (e.g. end minor, broadband Bragg reflector, etc.), yields a clearly visible peak on the OTDR trace. Hence, when testing with an OTDR, the length of the termination fiber can in principle be determined, thereby allowing determination of which termination fiber (R1 to R4) is present.
A limitation of this technique is that, when the connector between the fiber end and the receive box has a very low reflectance and loss, such is generally the case when an APC (Angled Physical Contact) type of connector is used, the reflectance of such connectors, when mated, is often negligibly small and, hence, undetectable with an OTDR. The detection of a reflectance “event” at the distal end of the FUT, yielding a peak on the OTDR trace, provides an important reference point for calculating the end of the termination fiber. If the connection between the fiber end and the termination fiber box cannot be detected, the length of the termination fiber often cannot be reliably determined, which makes it very difficult to verify that the fiber end is properly mapped, as well as rendering the FUT loss measurement less precise.